It has been demonstrated that cholesterol is depleted from the sperm cell plasma membrane during capacitation of epididymal rat spermatozoa, under chemically defined conditions. On the other hand, vesicles (synthetic and natural) with decapacitation activity elevate the sterol concentration in the membrane. We propose to investigate in further detail the effect of these changes on the physical properties of the sperm plasma membrane. It has also been proposed to investigate the effects of model synthetic vesicles of defined composition on the process of sperm capacitation. The effect of recently discovered changes in the composition of uterine fluid will be examined in terms of their effects on sperm capacitation.